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Nociceptors: the sensors of the pain pathway
Adrienne E. Dubin1 and Ardem Patapoutian1,2
1Department
2Genomics
of Cell Biology, The Scripps Research Institute, La Jolla, California, USA.
Institute of the Novartis Research Foundation, San Diego, California, USA.
Specialized peripheral sensory neurons known as nociceptors alert us to potentially damaging stimuli at the skin
by detecting extremes in temperature and pressure and injury-related chemicals, and transducing these stimuli
into long-ranging electrical signals that are relayed to higher brain centers. The activation of functionally distinct
cutaneous nociceptor populations and the processing of information they convey provide a rich diversity of pain
qualities. Current work in this field is providing researchers with a more thorough understanding of nociceptor cell
biology at molecular and systems levels and insight that will allow the targeted design of novel pain therapeutics.
Introduction
Pain, as a submodality of somatic sensation, has been defined as a
“complex constellation of unpleasant sensory, emotional and cognitive experiences provoked by real or perceived tissue damage and
manifested by certain autonomic, psychological, and behavioral
reactions” (1). The benefit of these unpleasant sensations, however, is underscored by extreme cases: patients lacking the ability
to perceive pain due to hereditary neuropathies often maintain
unrealized infections, self mutilate, and have curtailed life spans
(2). Normally, nociception (see Glossary, Sidebar 1) and the perception of pain are evoked only at pressures and temperatures
extreme enough to potentially injure tissues and by toxic molecules and inflammatory mediators. These high threshold physical
and noxious chemical stimuli are detected by specialized peripheral sensory neurons (nociceptors). This is in contrast to the high
sensitivity of visual, auditory, olfactory, taste, and somatosensory
organs to their adequate stimuli. Pain is described as having different qualities and temporal features depending on the modality
and locality of the stimulus, respectively: first pain is described
as lancinating, stabbing, or pricking; second pain is more pervasive and includes burning, throbbing, cramping, and aching and
recruits sustained affective components with descriptors such as
“sickening” (3). The intensity of these global reactions underscores
the importance of avoiding damaging situations for survival and
maintaining homeostasis. As opposed to the relatively more objective nature of other senses, pain is highly individual and subjective
(4, 5) and the translation of nociception into pain perception can
be curtailed by stress or exacerbated by anticipation (6).
Here, we review the nociceptive aspect of pain perception, focusing on nociceptors innervating the skin and subserving exteroception of noxious stimuli. Discussion of the similarities and differences among cutaneous, visceral, muscle, and joint nociception
can be found elsewhere (7–9). We provide an overview of how noxious stimuli are detected, encoded, and conveyed to the CNS. Since
recent reviews have described in detail the molecules involved in
detecting noxious stimuli (10–13) and contributing to protective
mechanisms mediating enhanced pain at the site of injury (14), we
take an integrative approach that highlights recently discovered
cellular transduction/conduction mechanisms in the context of
different nociceptor fiber types identified in vivo and ex vivo. We
further discuss innovations using genetic and pharmacological
tools that begin to address how particular nociceptor populations
Conflict of interest: The authors have declared that no conflict of interest exists.
Citation for this article: J Clin Invest. 2010;120(11):3760–3772. doi:10.1172/JCI42843.
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contribute to the perception of specific pain qualities. Since maladaptive changes in normal physiological mechanisms underlie a
variety of pathologies leading to chronic pain, a thorough understanding of nociception is required to identify the interventions
most likely to provide therapeutic benefit.
Anatomy and physiology of cutaneous nociception
Significant insights into the cellular and molecular basis of cutaneous nociception have been realized from studies on conscious
humans and surrogate animal models (15, 16), although we are far
from understanding the cell biology of pain perception. Advances
are hampered by the difficulties inherent in studying neuronal processes in humans, cellular changes in nociceptors induced by invasive methods, the inability to record directly from the tiny structures
where transduction of noxious stimuli occurs, and the uncertainty
in model systems that an animal’s behavior is due to its perception of
pain (15, 17). Although the morphology of sensory nociceptive nerve
endings is highly conserved in animals from rodents to humans (5, 9, 17–19), cutaneous nociceptors are an extremely heterogeneous
group of neurons housed in peripheral sensory ganglia located just
outside the CNS that transduce external noxious stimuli in the skin,
up to meters away from their cell bodies.
Minimally invasive extracellular single unit recordings from
nerve fibers in peripheral nerves (microneurography) and skinnerve preparations in mammals (20) and microneurography
combined with psychophysical measurements in human subjects (15, 16, 21) have revealed the existence of distinct classes of nociceptor activated by noxious stimuli. Adequate stimuli include
temperature extremes (> ~40°C–45°C or < ~15°C), intense pressure, and chemicals signaling potential or actual tissue damage.
Nociceptors are generally electrically silent (12) and transmit allor-none action potentials only when stimulated. However, nociceptor activity does not per se lead to the perception of pain. The
latter requires peripheral information to reach higher centers and
normally depends on the frequency of action potentials in primary
afferents, temporal summation of pre- and postsynaptic signals,
and central influences (7).
The speed of transmission is directly correlated to the diameter of
axons of sensory neurons and whether or not they are myelinated.
Most nociceptors have small diameter unmyelinated axons (C-fibers) (12) bundled in fascicles surrounded by Schwann cells and support
conduction velocities of 0.4–1.4 m/s (22) (Figure 1). Initial fast-onset
pain is mediated by A-fiber nociceptors whose axons are myelinated and support conduction velocities of approximately 5–30 m/s
(most in the slower Aδ range) (22). Nociceptive fibers have been
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Sidebar 1
Glossary
Action potential, actively propagating electric signal along neuronal processes that enables communication to postsynaptic
neurons in the pain pathway
Allodynia, pain evoked by a normally innocuous stimulus
Antidromic, transmission of action potentials against the peripheral to central direction
Axon reflex, the invasion of orthodromic (afferent) action potentials into other terminal branches of the same axon innervating the skin (antidromic, efferent) can cause the release of proinflammatory substances (e.g., peptides) from those peripheral
terminals to have a local effect on nearby terminals and nonneuronal components including the vasculature
Central sensitization, changes in synaptic efficacy between sensory neurons and central (spinal dorsal horn) neurons that
increase the perception of pain, especially in regions adjacent to the primary site of injury
Fatigue, a decrement in response to repeated stimuli applied to the receptive field of sensory cell
Hyperalgesia, an increase in the perception of pain elicited by a noxious stimulus; Hyperalgesic, a treatment that increases
the sensitivity to noxious stimuli; Primary hyperalgesia, increased pain perception in area of injury or exposure to noxious
stimulus; Secondary hyperalgesia, increased pain perception in the region surrounding the area exposed to a noxious
stimulus. Can be associated with a decrease in threshold and/or increase in suprathreshold response
Inflammatory pain, pain associated with tissue injury and inflammation characterized by reduced threshold and increased
responsiveness
Low threshold A-fibers, sensory neurons that innervate nonneuronal Merkel cells (Aβ-LTM SA I), Meissner corpuscle (Aβ-LTM FA I), Ruffini corpuscle (sensitive to stretch; Aβ-LTM SA II), pacinian corpuscle (vibration; Aβ-LTM FAII), and hairs
(hair movement; G1 hair (Aβ-LTM FA1), and Down (D-) hair (Aδ-LTM))
Mechanical stimulation, punctate mechanical stimulation: measuring reflex responses to constant force applied by calibrated filaments (often by Von Frey hairs). Pinch: Randall-Selitto increases pressure to the paw or tail via a clamp system. 1 bar = 14.5l psi = 0.987; atm = 750 mm Hg (Torr) = 100,000 Pa (N/m2) = ~10 g/mm2. Ambient pressure on the skin at sea
level is 1 bar (~10 g/mm2)
Myelinated neurons, neurons whose axons are wrapped within a membrane-rich myelin sheath that enables the action potential
to jump between axonal nodes of Ranvier (myelin-free areas) to increase the speed of its propagation (conduction velocity)
Neurogenic inflammation, a general term used to describe the effects of the local release of inflammatory mediators such as
substance P and CGRP from afferent nerve terminals
Nociception, the neural process of encoding and processing noxious stimuli
Nociceptor, a peripherally localized neuron preferentially sensitive to a noxious stimulus or to a stimulus that would become
noxious if prolonged; capable of encoding stimulus intensities within the noxious range; may have a wide dynamic range
of thresholds from innocuous to noxious range but stimulus-response relationship peaks in the noxious range; these also
include those that are not activated immediately but become responsive to heat and mechanical stimulation upon prolonged
stimulation
Nocifensive behaviors, “signs” of pain including “pseudoaffective reflexes” (e.g., withdrawal, jumping, contractures) and
conditioned motor responses (e.g., escape, avoidance, aggressiveness). Immobility (a behavior that could allow the animal to
preserve a painless posture) is often not scored (15)
Noxious stimuli, damaging or potentially damaging stimuli including extremes of temperature, mechanical stimulation, and
allogens that provoke an avoidance response
Nocisensor, a molecule that detects noxious stimuli and transduces the stimulus to a cellular response
Plasticity, a dynamic modulation of signaling
Receptive field (cutaneous), area of skin on which stimulation leads to response of a particular sensory neuron
Receptor potential, the change in membrane potential that occurs in the peripheral terminal due to transduction of a stimulus into ion flux, either through direct and/or indirect (through recruitment of second messenger cascades) modulation of
ion channel activity
classified (23) on the basis of their conduction velocity and sensitivity and threshold to noxious mechanical (M), heat (H), and cold
(C) (Tables 1 and 2) (9, 23). Units responding to thermal, mechanical, and chemical stimuli (polymodal) are the most common C-fiber type observed in fiber recordings (C-MH, C-MC, C-MHC) (7, 9, 24) (Table 1). The expression of differential repertoires of
transduction molecules (particularly chemical sensors) confers a
rich functional heterogeneity upon this class. The use of electrical stimulation to search for receptive fields, rather than mechani
cal stimulation, identified nociceptors normally insensitive to
mechanical and heat stimulation (silent; C-MiHi) that become
sensitive to noxious mechanical or temperature only after being
sensitized by inflammatory mediators (25, 26). C-fibers responsive
to noxious heat (C-H; ~10% of C-nociceptors) play a major role in
heat sensation (see below). A-fiber nociceptors are predominately
heat- and or mechanosensitive (A-MH, A-H, A-M) (19, 27) (Table 2); however, sensitivity to noxious cold is also observed (27–29).
Determining the contribution of each of these fiber types to pain
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Figure 1
Anatomy of nociceptors. (A) Somatosensory neurons are located in
peripheral ganglia (trigeminal and dorsal root ganglia) located alongside the spinal column and medulla. Afferent neurons project centrally
to the brainstem (Vc) and dorsal horn of the spinal cord and peripherally to the skin and other organs. Vc, trigeminal brainstem sensory
subnucleus caudalis. (B) Most nociceptors are unmyelinated with
small diameter axons (C-fibers, red). Their peripheral afferent innervates the skin (dermis and/or epidermis) and central process projects
to superficial laminae I and II of the dorsal horn. (C) A-fiber nociceptors
are myelinated and usually have conduction velocities in the Aδ range
(red). A-fiber nociceptors project to superficial laminae I and V.
perception requires an understanding of the molecular mechanisms underlying the detection of particular stimulus modalities
and nociceptor connectivity in central circuits.
Noxious stimuli are transduced into electrical signals in free
“unencapsulated” nerve endings that have branched from the
main axon and terminate in the wall of arterioles and surrounding
connective tissue, and may innervate distinct regions in the dermis
and epidermis (17, 30). The endings are ensheathed by Schwann
cells except at the end bulb and at mitochondria- and vesicle-rich
varicosities (17). A–fibers lose their myelin sheath and the unmyelinated A-fiber branches cluster in separated small spots within a
small area, the anatomical substrate for their receptive field (17).
C-fiber branches are generally more broadly distributed, precluding precise localization of the stimulus (17). In contrast, special3762
ized nonneuronal structures conferring high sensitivity to light
touch, stretch, vibration, and hair movement are innervated by
low threshold A-fibers (11). Nociceptive endings are in the vicinity
of keratinocytes, mast cells, and Langerhans cells, indicating the
capacity of peripheral sensory endings to monitor the status of the
skin (31). Nociceptors, like other primary somatosensory neurons,
are pseudounipolar (Figure 1): a single process emanates from the
cell body in the dorsal root ganglion (DRG) or trigeminal ganglion
(TG) and bifurcates, sending a peripheral axon to innervate the
skin and a central axon to synapse on second-order neurons in the
dorsal horn of the spinal cord or the trigeminal subnucleus caudalis (Vc) (13), respectively (Figure 1A). In this way, propagating
electrical signals between periphery and spinal cord (or brainstem)
follow a direct axonal pathway, thus reducing the risk of conduction failure (32). Nociceptors are excitatory neurons and release
glutamate as their primary neurotransmitter as well as other components including peptides (e.g., substance P, calcitonin generelated peptide [CGRP], somatostatin) important in both central
synaptic signaling and efferent signaling in the skin (13). Invasion
of action potentials into the nociceptor soma via the short stem
axon (32) can lead to biochemical changes (e.g., phosphorylation
and activation of MAPK superfamily of signaling pathways) that
ultimately alter gene expression and functional phenotype (33, 34). Although it is thought that direct communication between the
soma of primary sensory neurons does not occur, vesicle exocytosis is observed in dissociated soma and may influence associated
Schwann cells and possibly nearby neurons (35, 36). The central
axon of DRG neurons enters the spinal cord via the dorsal root
and sprouts branches that innervate multiple spinal segments in
the rostral and caudal direction as well as the segment associated
with the particular DRG and dorsal column nuclei of the caudal
medulla (7). They terminate predominantly in laminae I, II, and V
of the dorsal horn on relay neurons and local interneurons important for signal modification (13, 37, 38) (Figure 1, B and C). The
relay neurons project to the medulla, mesencephalon, and thalamus, which in turn project to somatosensory and anterior cingulate cortices to drive sensory-discriminative and affective-cognitive
aspects of pain, respectively (38). Local inhibitory and excitatory
interneurons in the dorsal horn as well as descending inhibitory
and facilitatory pathways originating in the brain modulate the
transmission of nociceptive signals, thus contributing to the prioritization of pain perception relative to other competing behavioral
needs and homeostatic demands (39).
The cell body (soma) has served as an extremely useful model
to study molecules and modulatory mechanisms mediating
transduction of noxious stimuli, transmission of electrical signals
to the CNS, and release of neurotransmitters and neuropeptides
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Table 1
Major heat and/or mechanosensitive nociceptor C-fiber classes
Mechanical sensitivity
Mechanical insensitivity
References
C-MH
C-M
C-H
35–approaching 100
10–15
5–10
10–25
25, 50, 67
0.81 (human); 0.87 (monkey); 0.35 (mouse); slower
than mechanically sensitive C-fibers)
0.80 (human); 0.82 (monkey); slower than mechanically
sensitive C–fibers
25, 50, 55,
123, 124
Unknown
NA
24, 67, 106
Heat threshold
39°C–51°C
NA
42°C, 48°C (>C-MH)
NA
50, 55, 67,
123, 124
Percentage observed
CV (m/s)
0.8–1.0 (human, monkey);
0.84 (human); ~0.5 (mouse)
~0.5 (mouse
Correlation of heat response and PR
Yes/repeated stimulation sensitizes
(hairy, not glabrous)
NA
NA
C-MiHi (“silent”)
Correlation of mechanical response to punctuate
stimulation and PR
Static: no; accelerating: yes
Accelerating: yes
NA
Mechanical threshold
30 mN (human); 25 mN (monkey); 17 mN (mouse)
30 mN (human); High
High
18 mN (mouse)
24, 125
25, 26,
50, 55
Correlation of tonic
Yes; initial pain; Yes; initial pain; pinch response to adapting
adapting
tonic pinch and PR
Yes: slow development of pain; sensitization; perhaps indirect
Yes: slow development of pain; sensitization;
perhaps indirect
9, 18, 126
Electrical stimulation
No: uninjured skin
No: uninjured skin
of axon → flare
Yes: UV-B–treated skin
Yes: restricted to receptive field
Yes
127–130
Cap → TRPV1 (prolonged); AITC → TRPA1, 32%; MH sensitivity unmasked
by CAP; histamine, BK, PGE2
16, 25, 60,
124, 131
Known chemical activators
Cap → TRPV1 (transient); AITC → TRPA1 Cap → TRPV1 AITC → TRPA1 (58%); (36%)
(prolonged); histamine, diversity of chemical BK, PGE2
activators: e.g., pH)
Effect of injury/
Nonuniform changes in Unknown
inflammation
activity after burn: hypoalgesia (glabrous); sensitization (hairy)
Inflammation: hyperalgesia; sensitive to mechanical stimuli after injection of inflammatory soup; BK: sensitization
Thresholds for mechanical and heat stimuli in “sensitive” range; sensitization to
suprathreshold stimuli
26, 60, 106,
123, 124,
128, 132
Comments
Responsible for TRPV1-heat hyperalgesia; not capable of precise localization of stimulus
May be “C-H” fibers with higher heat thresholds
or peripheral endings
deeper in the skin
50, 124
Heat responses independent of TRPV1; may provide neural code for magnitude judgments of heat pain; contribute to stimulus localization
Classification of nociceptors on the basis of mechanical sensitivity varies among investigators using different cut-off values and methods. For clarity, coldsensitive fibers are not included. Cap, capsaicin; CV, conduction velocity; PR, human subject pain ratings; +, immunoreactive.
at central and peripheral terminals (40, 41). The soma expresses
many molecular entities that are expressed in free nerve endings,
central terminals, and axon (13). However, data from whole-cell
soma recordings have been shown in a few cases to be at odds with
behavioral or peripheral physiological data (e.g., heat transduction,
refs. 42–44; and proton responsiveness, ref. 45). Although the
underlying differences in these cases may be due to differential
distribution of transduction molecules, it is also possible that
nonneuronal peripheral components are required in vivo and lacking in dissociated neuronal cultures. This underscores the importance of corroborating results from cultured neurons with behavior and/or acute preparations retaining intact terminal fields.
Labeling with retrograde dyes injected into the target tissue has
enabled characterization of functional attributes of the soma of
nociceptors innervating those tissues. The heterogeneity of functional phenotypes observed in isolated sensory cell bodies (46, 47)
appears to reflect the variability observed in cutaneous nociceptor
fiber types observed in studies in which recordings from fiber or
soma during receptive field stimulation are combined with subsequent nociceptor labeling to identify terminal morphology (48, 49) and the expression of nocisensors (50), markers, and peptides (48)
(Table 1). Nociceptors differentially express a variety of anatomical and biochemical markers (e.g., the expression of versican, the
binding partner for the isolectin B4 [IB4]; ref. 51), however the
functional significance of these markers, especially given striking
species differences (49, 52), are unknown. Here, we will address
how the functional heterogeneity of the nociceptor has an impact
on the perception of pain.
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Table 2
Major heat and/or mechanosensitive nociceptor A-fiber classes
A-MH II
A-MH I
A-M
References
Percentage observed
~10 (hairy skin)
~20 (hairy skin)
15–50
50, 55
CV (m/s)
11, 15 (monkey)
11, 25 (monkey)
14 (monkey)
55, 57
Yes; response is transient (rapid onset [<1 s], early peak discharge [<1 s], adapting); fatigue to repeated stimuli
Yes: long latency to respond and late peak
discharge; shows
sensitization
NA
55
43°C–47°C
Yes but with threshold offset
>53°C
YES
NA
YES
55
78, 132
8.2 bar (monkey); 25 mN (mouse)
50, 55, 59
Correlation of heat response and PR
Heat threshold
Correlation of mechanical response to punctuate stimulation and PR
Mechanical threshold
11–15 bar (monkey)
3.7–5 bar (monkey)
Cap insensitive
Cap insensitive
57, 59
Effect of injury/inflammation
Unknown
Known chemical activators
Cap → TRPV1
Burn: heat sensitization, hyperalgesia in glabrous skin; may mediate 2° punctate mechanohyperalgesia
Punctate hyperalgesia; may mediate 2° punctate mechanohyperalgesia
19, 55,
130, 133
Comments
Mediates 2° punctate hyperalgesia due to central sensitization; May contribute to slowly developing pain to heat
Mediates 2° punctate hyperalgesia due to central sensitization; Most TRPV2+
but insensitive to 54°C;
HTMs
50, 55,
78, 130
Mediates first pain to heat and provides precise localization of stimulus; not observed in glabrous skin; eliminated in skin desensitized by Cap
For clarity, A-H and A-MiHi (“MIA,” ref. 26), which give long-lasting responses to capsaicin (57), are not included in this table. HTM, high threshold mechanoreceptor.
The response of nociceptors to noxious stimuli
Close proximity of distal extremities to a hot or cold surface, intense
pressure or squeezing, and irritating chemicals can result in a subsecond somatopic withdrawal response. Activation of nociceptors
requires that adequate stimuli depolarize peripheral terminals (producing a receptor potential) with sufficient amplitude and duration.
This ensures that despite any attenuation and slowing of the receptor
potential by passive propagation between the sites of transduction
and action potential generation, information such as stimulus intensity will be encoded in the resulting train of impulses. Although the
distance to action potential initiation is not known for fibers innervating the skin, action potential generation has been proposed to be
at or near the site of transduction in Aδ cold fibers innervating the
guinea pig cornea (53). In this model, action potentials can be generated at differing distances from the terminal ending depending on
the extent of depolarization of the fiber and resulting inactivation
of voltage-gated channels involved in conduction (53). Theoretically,
the depolarizing receptor potential can be accomplished by multiple
membrane conductance changes and electrogenic pump activity.
Since the electrochemical gradients for sodium (Na+), calcium (Ca+2),
and chloride (Cl–) (37) are more positive than the resting membrane
potential in sensory neurons, the opening of ion channels permeable to these ions will cause the membrane potential to shift in the
positive direction (depolarize). Since the electrochemical gradient
for potassium (K+) is more negative than resting potential, closure
of active potassium channels not only depolarizes the membrane
potential but amplifies current-induced voltage fluctuations due to
the resulting increase in membrane resistance (54).
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The identification of the molecules that mediate or contribute to
heat-, cold-, mechanical-, and chemical-induced generator potentials has been achieved by careful characterization of currents in
native cells, the use of novel approaches to identify genes encoding
channels and receptors, genome sequencing and bioinformatics,
chemical tools (e.g., calcium dyes, pharmacological agents), and
innovative genetic approaches. Although significant progress has
been made in the study of heat and chemical transduction, many
pieces of the puzzle are still missing (Table 3 and Figure 2).
Transduction of noxious heat
At least 5 classes of nociceptor reveal an increase in activity dependent on the intensity of the heat stimulus beyond the threshold
for pain perception (~40°C–45°C) into the noxious range (9, 24).
Microneurography studies on conscious human subjects have
addressed whether activity in a particular fiber correlates with the
psychological rating of pain. Under normal conditions, the activity
in only a subset of heat-responsive fibers correlates to the degree of
pain perceived (C-MH, A-MH type I, A-MH type II; Tables 1 and 2). A-fibers (A-MH Type II innervating hairy skin) responding to
temperatures slightly cooler than the perceptual pain threshold
for heat are proposed to mediate first pain in humans (55). These
fibers rapidly activate, adapt during prolonged heat stimulation,
fatigue between heat stimuli, and are sensitive to capsaicin (55–57), a selective agonist of the mammalian heat-activated nonselective
cation (NSC) channel transient receptor potential VI (TRPV1)
(58) (Table 3). In humans and monkeys, slowly developing second
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(9, 24), the latter requiring longer exposures to noxious temperatures to achieve maximal firing rates (Tables 1 and 2). Heatinduced C-MH fiber activity in primates correlates with human
pain perception in the absence of injury (9). In human glabrous
skin, however, neither first pain (56) nor A-MH type II–fiber
innervation is observed (59). Investigation of noxious heat
transduction in animal models is complicated because the conservation of these pathways is unknown.
Although human C-MH polymodal nociceptors are activated in
a temperature range (39°C–51°C) similar to recombinant TRPV1
(58) and have been reported to be transiently activated by capsaicin (60), genetic deletion of TRPV1 in mice only partially reduces
noxious heat sensitivity in behavioral assays (42, 43) and has no
effect on heat responsiveness of the C-fibers tested (42, 44, 49). This suggests that, at least in mice, heat sensors other than
TRPV1 are the major contributors to acute heat-induced pain.
Cultures of rodent sensory ganglia indicate expression of TRPV1
in approximately 50% of neurons (61) and 75% of small-medium
diameter neurons (43), similar to the proportion of C-MH fibers
in a skin-nerve-sensory ganglion–spinal cord preparation (49).
Although functionally characterized mouse C-MH neurons were
not immunoreactive for TRPV1 (50), neurons with low expression levels may have been missed. Importantly, every identified
mouse C-H neuron (~10% of the population) revealed TRPV1-like
immunoreactivity (50) and no functionally identified C-H nociceptors were observed in TRPV1-deficient animals. This result
cannot be explained by a phenotypic switch to C-MH fibers in
these mice since mechanical sensitivity of the C-fiber population
was unchanged (50). Furthermore, noxious acute heat responses
evoked in rat C-MH saphenous fibers were not decreased by ruthenium red at concentrations that block the capsaicin response (45).
In humans, the prolonged time course of the strong C-H response
to capsaicin applied to the receptive field matched the duration of
the perceived pain (60) and is consistent with the results of rodent
studies (50). Disparities regarding the contribution of TRPV1 to
fiber subtype functionality (especially C-MH fibers) may depend
on species, stimulation techniques, tissue preparation, and sensitivity of the assay. However, it is clear that acute heat responses
are mediated by mechanisms in addition to TRPV1 activation,
at least in mouse. In the context of tissue injury, however, polymodal C-MH fibers are certainly important in TRPV1-mediated
thermal hyperalgesia, indicating that inflammation upregulates
the contribution of TRPV1 to heat-evoked nocifensive behaviors
in mouse (see below). TRPV2 (Table 3) has been suggested to
mediate, at least in part, the receptor potential in A-MH type I– fibers, since it is activated by temperatures greater than 52°C in
recombinant expression studies (62); however, the lack of selective pharmacological tools for manipulation of TRPV2 activity
has hindered rigorous evaluation of this hypothesis. Transducers of heat stimuli into the noxious range other than TRPV1 and
TRPV2 (45, 49) include other TRPV channels (TRPV3 and TRPV4)
that have been reported to exist on sensory neurons (10) as well as
constitutively active K+ channels (KCNK2) inhibited by heat (63)
(Table 3). Although TRPV3 undergoes dramatic sensitization to
repeated heat stimuli, TRPV3 does not appear to be involved in
agonist sensitization of C-fiber heat responses (44). Nocisensors
directly activated by heat are also expressed on resident skin cells
(e.g., TRPV3 and TRPV4 on keratinocytes; refs. 10 and 31) and
cause the release of allogenic substances that indirectly influence
nociceptor firing (64) (Table 3).
Transduction of noxious cold
Identifying the mechanisms used by nociceptors to transduce
noxious cold has lagged behind progress in understanding heat
transduction mechanisms (65). The intensity of cold pain in
humans increases linearly with stimulus intensity between about
20°C and 0°C. The threshold for pain perception to cold is much
less precise than that for heat, but is about 15°C (66). There is
tremendous variability in threshold for cutaneous cold-evoked
fiber activity observed in mammals in part due to the rate of cooling (approximately +30°C to –18°C; refs. 27, 66–68). Homeostatic
processes engaged during in vivo studies (e.g., vasculature changes) and potential tissue damage occurring at subfreezing temperatures are likely to indirectly influence nociceptor responsiveness.
Furthermore, measuring cold nocifensive behavior in animals has
proven to be challenging, perhaps due to the prolonged exposure
time in most assays (68).
Cooling the skin to 4°C activates A- and C-fibers sensitive to
innocuous cooling and cold-sensitive nociceptors (27–29, 67, 69),
consistent with the presence of two populations of cold-sensitive
neurons observed in culture (70–72). Cool-sensitive nonnociceptive afferents are spontaneously active at normal skin temperature and their excitability increases with decreasing temperature
(53, 67). The menthol-activated NSC channel TRPM8 (10) is
responsible for the detection of innocuous cooling (69, 73, 74)
and contributes to spontaneous firing (69) in mice. The effective
range for TRPM8-mediated cold coding extends from just below
skin temperature into the noxious range (10°C–15°C and below;
ref. 10). Although mouse studies have yielded conflicting results
regarding the requirement of TRPM8 in behavioral responses to
noxious stimuli, more recent work using a novel cold-plate assay
convincingly demonstrates a role for mouse TRPM8 in sensing
noxious cold (75). Furthermore, the analgesic effects of cold temperature (17°C) were lost in mice lacking TRPM8 in the context
of formalin-induced inflammation (74). Whether TRPM8-mediated analgesia is dependent on peripheral and/or central sites
of action is unknown but may be addressed now that TRPM8expressing neurons and their peripheral and central fibers can
be visualized by GFP expression driven by the TRPM8 promoter
(76, 77). Importantly, studies in humans and mice reveal species
differences in pathways sensing innocuous cold: A-fiber block
completely suppresses the cold response in humans (78), yet the
majority of TRPM8-expressing fibers responsible for innocuous
cold transduction in mice have small diameters (76, 77)
Noxious cold stimuli activate NSC currents and calcium influx
(10, 79) and decrease K+ channel activity (80) and Na+/K+-ATPase
function (65) (Table 3). The temporal dissociation of the qualities
of pain/ache vs. prickle/heat to noxious cold (3°C) suggest underlying differences in transduction mechanisms or information processing (66). The cation channel TRPA1 (10) has been proposed to
play a role in this process because it has a threshold near 17°C, is
expressed in nociceptors together with TRPV1 (10), and is required
for cold sensation in mice (81, 82). Human genetic studies have
suggested TRPA1 contributes to variation in cold-pain sensitivity (5). Although TRPA1 may respond to cold indirectly through
cold-induced intracellular calcium release (10), slow temperature
ramps can activate TRPA1 in excised patches in the absence of calcium (82, 83). However, the contribution of TRPA1 to cold sensation is debated since TRPA1 activation was not observed in a
heterologous expression system or cultured TG neurons to which
relatively short cold stimuli to 5°C were applied (84), and not all
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Table 3
Proposed ion channel sensors of cutaneous noxious stimuli
Noxious stimulus
Transduction channel
Evidence for role in acute nociception
Heat ≥ 43°C
TRPV1 ≥ 43°C Chemical activation (Cap, (unsensitized); resiniferatoxin); heat nociception specific receptor by activating C-H fibers (mouse); reduced central wide dynamic for sub-μM Cap
range neuron response to 49°C in null mice
TRPV2 ≥ 52°C, recombinant
Role in hyperalgesia
Comments
Refs
Major contributor to thermal (heat) after carrageenan or CFA-induced inflammation; heat threshold decreased by inflammatory mediators (e.g., BK, NGF-mediated phosphorylation via P–p38); after burn, heat hyperalgesia
reduced in KO; upregulation and
enhanced activity in inflammation
Only heat receptor that releases CGRP and substance P in periphery when activated; Cap induces predominantly
stinging and burning pain
10, 34,
42–44,
50, 134
Proposed to mediate heat response in A-MH type II–fibers
(human/primate)
49
Reveals strong sensitization by repeated heating; mediates ATP
release from keratinocytes and
PGE2 release from keratinocytes
overexpressing TRPV3
64, 135–138
Protein has been detected in DRG that depends on an intact TRPV4 gene; high expression in keratinocytes
10, 67,
139–142
2-pore potassium channel reduced activity causes increased excitability; contributes to suppression of
heat activation of C-MH fibers;
negatively regulated by Gs and Gq
protein–coupled receptors, such
as PGE2, serotonin, and glutamate
63, 143,
144
Two pore potassium channel
144
Predominant detector of innocuous cold in vivo; responsible for the analgesic effect produced by cold or chemical cooling compounds
10, 69,
73–75,
145
Controversial (see text); does not contribute to heat hyperalgesia during CFA-induced inflammation
10, 75, 81,
82, 85,
146–148
Combination of TREK-1 and TRAAK important
144
Hypertonic (10%) saline-
induced pain reduced
in null mice
63
Combination of TREK-1 and
TRAAK important
144
Although activated by hypotonicity, there is lack of consensus for a role
of TRPV4 in noxious mechanical stimulus
transduction; protein has been detected in
DRG that depends on an intact TRPV4 gene
142, 149
A direct effect of mechanical stimuli on mammalian TRPA1 has not been demonstrated
81, 147,
148
No evidence for a loss of heat ND
sensitive fibers in mice lacking TRPV2 together with TRPV1
TRPV3; threshold Heat: increased withdrawal Null mice reveal no deficits in thermal (heat) or mechanical 32°C–39°C into latency on hot plate ≥ 50°C
noxious range
hyperalgesia induced by CFA or BK and formalin-induced behaviors
TRPV4; threshold No contribution to 50°C-induced 27°C–34°C into spike frequency at 40°C; noxious range
no phenotype on hot plate to 50°C
Thermal (heat): slight increase in withdrawal latency at 45°C–46°C; activated by concerted action of components in inflammatory soup
KCNK2 (TREK–1) Null mice reveal: enhanced heat-
Recombinant channel activity inhibited 30°C–45°C
dependent (30°C–45°C) saphenous by H+, LPA; Inflammation increased C-fiber activity; decreased thermal heat (46°C) (but not cold) hyperalgesia (46°C–50°C) tail withdrawal latency; in C-MH fibers in null mice; LPA and higher percentage of heat-
protons inhibit channel activity
sensitive/TRPV1-negative fibers
KCNK4 (TRAAK)
Null mice reveal: higher percentage of heat-sensitive/TRPV1-negative fibers and heat hyperalgesia
Null mice reveal heat (46°C) hyperalgesia after carrageenan
Cold
TRPM8; threshold near 26°C and active into
the noxious range
Severe deficit in cooling-induced Contributes to cold allodynia behavior and sensory neuronal in CFA model of inflammation
responses in null mice; contributes to avoidance of moderately low temperature; contributes to avoidance
at noxious cold temperatures
and cold hypersensitivity
TRPA1; moderately specific receptor for mid-μM AITC
Contributes to nocifensive behaviors on 0°C cold plate and acetone-
induced nocifensive behaviors; and cold hypersensitivity at 10°C
KCNK4 (TRAAK)
Together with KCNK2 (but not alone) ND
contributes to noxious cold-induced
C-fiber activity (lowered threshold)
and behavior
Intense punctate KCNK2 and/or pinch (TREK-1)
pressure
Contributes to von Frey punctate mechanical–induced pain, but not pressure-induced pain
KCNK4 (TRAAK)
Contributes to CFA-induced cold hyperalgesia and BK-induced hyperalgesia; null mice reveal increased threshold to punctate mechanical stimuli
(von Frey) and reduced inflammatory hyperalgesia
Null mice reveal inflammation-induced mechanical (von Frey) hyperalgesia and reduced PGE2-induced pain
Contributes to punctate von Frey–
ND
induced pain; together with KCNK2 contributes to osmotic pain
TRPV4
TRPV4 KO: reduced response
Mechanical hyperalgesia
to tail clamp
TRPA1
Cutaneous nociceptor fibers from TRPA1-null mice exhibit lower firing rates in response to mechanical stimuli
TRPV1
Null mice have normal threshold for punctuate and pinch stimuli
Mechanical hyperalgesia BK-induced reduction in mechanical withdrawal
threshold required TRPA1
After burn, mechanical hyperalgesia Role in mechanical hyperalgesia
reduced in KO; BK-induced reduction in mechanical withdrawal threshold required
TRPV1; no defects in mechanical sensitivity
after AITC or CFA injection in null mice
42, 85,
134
Kv3.4
Mechanical hyperalgesia after i.t. ND
antisense ODN injection (rat lumbar DRG); no effect on heat response
Reduced activity causes
increased hyperalgesia
150
Kv4.3
Mechanical hyperalgesia after i.t. ND
antisense ODN injection (rat lumbar DRG); no effect on heat response
Reduced activity causes increased hyperalgesia
150
NGF, nerve growth factor; ND, not determined; i.t., intrathecal.
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Figure 2
Known or proposed transduction mechanisms
in uninjured mammalian nociceptor peripheral
terminals. Ion channels that transduce heat
(A), cold (B), and mechanical stimuli (C) are
depicted. Stimuli are presented to the skin,
which is depicted as containing representative nonneuronal cells (such as keratinocytes)
(brown cells) and the free nerve endings of
nociceptor axons (blue). Arrows next to channels indicate whether their activity is increased
or decreased upon stimulation. Note that these
nocisensors are not necessarily coexpressed
in the same terminal. The curved arrow in C
refers to the transducer(s) and other channels
and molecules that contribute to the firing pattern (e.g., rapidly adapting vs. slowly adapting) in these fibers. Molecularly unidentified
channels with indicated ion permeabilities
inside drawing of channel are referred to as
“putative RA MA channel” and “putative IA/SA
MA channel.” MA, mechanically activated;
RA, rapidly adapting; IA, intermediate adapting; SA, slowly adapting.
mouse lines constitutively lacking TRPA1 reveal a cold phenotype
in behavioral and neural assays (85). However, two independent
studies recently demonstrated a role of TRPA1 in noxious cold
sensitivity (75, 82). TRPA1 is established as a general sensor for
noxious irritating electrophilic compounds (including allyl iso
thiocyanate [mustard oil] [AITC] and cinnamaldehyde, the active
pungent ingredients in hot mustard and cinnamon, respectively;
refs. 84 and 86) and is sensitized by inflammatory mediators (87).
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(88, 89). Importantly, endogenous reactive chemicals are also
effective agonists of TRPA1 (90). How TRPA1-expressing neurons
might mediate the burning sensation of hot mustard (AITC) may
be explained by anatomical and psychophysical results: AITC is a
strong chemical activator of a subset of TRPV1-expressing neurons, and activity in peripheral fibers transmitting information
about cold stimuli in the presence of an A-fiber block (presumably
C-fibers) evoke burning, aching, and pricking qualities (67). Interestingly, the activation of some cold fibers by noxious heat may be
the basis for the paradoxical cold sensation felt by stimulating cold
spots with noxious heat stimuli (67).
Transduction of noxious mechanical stimuli
Whereas heat- and chemical-induced nociceptor responses correlate with pain perception in humans (9, 24), mechanical stimulation of C-MH (24) and rapidly adapting A-HTM (18) fibers may
not (24) (Tables 1 and 2). The perception of pinprick pain intensity,
however, is related to activity in capsaicin-insensitive A-fiber nociceptors (e.g., A-M and A-MH type I) (78). Transduction channels
mediating mammalian noxious (and innocuous) mechanical stimuli have been elusive (5, 91, 92). Transduction in soma membranes
on a submillisecond time scale suggests direct gating by pressure
of ion channels with NSC and possibly Na+ permeability (91). No
orthologs for the well-studied prokaryotic mechanosensitive channels MscL and MscS are present in mammalian genomes (93). In
addition, there is no strong evidence that mammalian orthologs
of invertebrate components of structures involved in mechanosensation (92) are the transduction channel in cutaneous sensory
neurons, although acid-sensitive ion channels (ASICs) appear to
play a role (11). Some progress has been made in the identification
of proteins (e.g., stomatin-like protein 3; ref. 94) involved in sensing innocuous touch, but their genetic deletion in mouse does not
compromise behavioral responses to noxious pressure. TRP channels are candidates based on expression and functional similarities to evolutionary counterparts, but whether these contribute to
the molecular sensor/transducer or function in a sensitizing role is
still unclear (11) (Table 3). The challenges encountered in assigning
a role for transducers of noxious mechanical stimuli include the
efficacy of stimulation protocols applied in behavioral and ex vivo
tissue assays (e.g., phenotypes can differ when challenged with thin
calibrated von Frey nylon filaments vs. distributed pinch that may
also induce local ischemia; ref. 95), the suppressive influence of
innocuous A-fiber mechanosensitive inputs at a systems level (18),
and the extrapolation of cellular assays to nociception (e.g., poking,
ref. 96; stretching, ref. 97; hypo-osmotic–induced swelling, ref. 98).
This topic is reviewed in more detail elsewhere (12, 19, 91–93).
Conduction
Nociceptors express a wide variety of voltage-gated channels (e.g.,
Nav, Cav, Kv) that transduce the receptor potential into an action
potential or, more commonly, a set of action potentials that
encode the intensity of a noxious stimulus applied within their
receptive fields. There are 9 known Nav, 10 Cav, and 40 Kv genes
in mammals (http://www.iuphar-db.org/DATABASE/), many of
which have multiple splice variants with different functional characteristics (99). Cell excitability and firing behavior (e.g., threshold
for action potential generation, action potential and undershoot
amplitude and duration, and maximal firing frequency) depend
on the complement of these channels as well as those contributing to frequency modulation (e.g., hyperpolarization-activated
3768
cyclic nucleotide-gated cation channel [HCN] and A-type Kv4.3
and Kv3.4 channels) (54). For instance, nociceptors responsive to
noxious cold require the expression of the tetrodotoxin-resistant
(TTX-resistant) Nav1.8 channel at the peripheral terminal (100),
and mice lacking Nav1.8 and Nav1.7 display deficits in mechanosensation (95, 101). Peripheral CGRP release by inflammatory
mediators is unaffected by TTX, suggesting an important role of
TTX-resistant Nav in regulated pain thresholds, consistent with
their robust modulation by bradykinin (BK) and PGE2 (102) (see
below). Since enhanced excitability of primary sensory neurons in
inflammatory and pathologic pain states is a major contributor
to the perception of pain, specific pharmacological agents that
specifically dampen aberrant activity are desirable in the design of
pain therapeutics. To this end, an understanding of species-specific differences is critical, as exemplified by the dramatically different phenotypes in mice and humans lacking Nav1.7: although
mice lacking Nav1.7 show a mechanosensory (pinch) and formalin-induced (5%) pain phenotype (103), humans lacking Nav1.7 are
insensitive to pain altogether (104).
Central modulation
Nociceptors release a variety of substances from their central terminals that have the potential of exciting second-order neurons
through multiple mechanisms. Fast and slow synaptic transmission are mediated in large part by glutamate and peptides (e.g.,
substance P, CGRP), respectively (7, 38). Of particular importance
to pain perception is the plasticity in synaptic strength (i.e., the
ability to enhance homosynaptic as well as heterosynaptic connections) between primary afferents and the relay and interneurons
they drive, presynaptic and postsynaptic modulation by descending facilitatory and inhibitory pathways in the spinal cord, and
the efferent aspects of nociceptor function activated by strong
GABAergic/glycinergic depolarization of presynaptic terminals
leading to the dorsal root reflex (37, 105). Anterograde transmission of action potentials from the spinal cord to the periphery
results in release of peptides and other inflammatory mediators
in the skin and exacerbates nociceptor excitability and pain (see
below). It is at the spinal level that nonnociceptive neurons are
recruited by strong nociceptor activation through functional
modulation of local circuits (105).
Adaptive and maladaptive shifts in pain threshold
Injury to the skin induces protective physiological responses
aimed at decreasing the likelihood of exacerbating the injury.
After an injury induced by pungent chemicals (e.g., capsaicin,
mustard oil) and burn, stimulation of the injured area produces
enhanced pain to noxious stimuli (primary mechanical and thermal hyperalgesia) dependent on C-fiber activity that manifests as
a decrease in threshold to activate C-MH fibers and to perceive
pain (9, 19, 106). Immediately surrounding the injured area, a
zone of flare (reddening) develops and stimulation of even a larger
secondary zone produces pain in response to normally innocuous
stimuli (e.g., brush stroke) (secondary mechanical allodynia) as
well as enhanced responsiveness to noxious mechanical (secondary mechanical hyperalgesia) and thermal (heat) hyperalgesia if
spatial summation is invoked (secondary thermal hyperalgesia)
(21, 105, 107). Here, noxious punctate stimulation of C-nociceptors induces secondary mechanical hyperalgesia mediated by
A-nociceptors (7) and innocuous dynamic mechanical stimuli
(gentle stroking) provokes nonnociceptor A-fiber–mediated pain
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(108). Cellular mechanisms underlying this complicated response
involve both peripheral and central processes (14, 38, 105, 107)
and require nociceptor input, particularly A-MH and C-MH fibers
(19, 91, 105). After a burn, A-MH fibers (most likely type I) mediate primary heat hyperalgesia in glabrous skin (9).
What are the cellular mechanisms mediating hyperalgesia? Electrical stimulation of the majority of C-polymodal fibers produced
plasma extravasation in their peripheral receptor field (109). Centrally propagating impulses can antidromically invade peripheral
arborizations innervating other areas in the afferent’s receptive
field (axon reflex), causing the release of peptides (e.g., substance
P, CGRP, somatostatin) and/or other bioactive substances from
the terminal (e.g., cytokines) into the interstitial tissue (17). The
released substances produce a myriad of autocrine or paracrine
effects on endothelial, epithelial, and resident immune cells
(Langerhans), which lead to arteriolar vasodilatation (“flare,” via
CGRP) and/or increased vascular permeability and plasma extravasation from venules (edema, via substance P). Liberated enzymes
(e.g., kallikreins) and blood cells (e.g., platelets, mast cells) further
contribute to the accumulation of inflammatory mediators and
neurogenic inflammation (110, 111). A large variety of substances
feed back onto nociceptors innervating the injured region and
sensitize peripheral terminals by direct and indirect actions at ion
channels, receptors, and second messenger cascades (87, 102, 111,
112). The mechanisms by which inflammatory mediators recruit
“silent” C-MiHi fibers by endowing these fibers with prolonged
mechanical and heat sensitivity (25, 60) is unknown but may
involve long-lasting alterations in second messenger signaling
cascades and sensitization of nocisensors. Decreased thermal and
chemical thresholds in the primary area are due in part to sensitization of TRPV1 and TRPA1 by numerous components of the
“inflammatory soup” including BK (36, 85–87, 112). Although
TRPV1 is not the only transducer of acute noxious heat, it is
the major contributor to the development of heat hyperalgesia,
perhaps due to its expression in polymodal neurons capable of
inducing neurogenic inflammation and modifying central connections (42, 43). As a polymodal nocisensor, TRPV1 is the target
for acute acid responsiveness in the skin of mice, although species
differences are apparent (113). A recently described phenomenon
(“hyperalgesic priming”) evoked by cytokine- and neurotrophininduced recruitment of Gi/o-PKCε signaling in nociceptors can
produce prolonged sensitization and mechanical hyperalgesia
and may contribute to chronic pain (114). Prolonged pain perception observed in inflammatory pain models is generally believed
to be produced by ongoing nociceptor activity (15); formalin, for
instance, produces nocifensive behaviors through its activation of
TRPA1 (115, 116). Secondary hyperalgesia to punctate pinprick
stimuli is mediated at least in part by capsaicin-insensitive A-fiber
nociceptors (e.g., A-MH type I and A-M; Table 2) by central sensitization processes (78, 108).
Do labeled lines transmit noxious stimulus information?
Determining the extent to which pain qualities are dependent on
the activation of subpopulations of neurons and intensity coding
poses a considerable challenge and is an active area of research
(13). Psychophysical studies on spinal cord injury patients suffering from partial or complete loss of thermal sensitivity support a
model in which both pain-specific pathways and nonnociceptive
pathways are integrated (117). Significant crosstalk between these
pathways exists at multiple levels including stimulus transduction
(118), peripheral terminals during neurogenic inflammation, and
central connections during central sensitization and may underlie
paradoxical temperature sensation. To address the extent by which
particular nociceptive signaling pathways encode particular modalities, a number of approaches have been taken. Genetically encoded tracers have enabled visualization of specific subpopulations of
sensory neurons (e.g., Mrgpr family, ref. 30; TRPM8, refs. 76 and 77) and determination of their innervation patterns as well as cellular
function. Pharmacologic and hereditary genetic ablations have
defined the role of nociceptors in pain but until recently have
included multiple or entire nociceptor populations (e.g., capsaicininduced ablation of TRPV1-expressing neurons; refs. 42 and 119)
or sensory and autonomic populations (2). Genetically mediated
ablation using toxins (e.g., diphtheria A) transcribed by promoters of genes expressed in specific neuronal subtypes (e.g., Nav1.8expressing neurons, ref. 120) provides a means to address the contributions of particular cells to acute nociception and hyperalgesia
and allodynia after injury.
Mice expressing diphtheria toxin under the Nav1.8 promoter
reveal significant loss of sensory neurons (85% and 13% of unmyelinated and myelinated neurons, respectively, consistent with
Nav1.8 expression in most nociceptors as well as some myelinated
sensory neurons). These mice are unresponsive to acute noxious
mechanical (pressure-induced, not punctate) and cold stimuli and
defective in the development of inflammatory pain, but are normal with regard to acute responsiveness to heat stimuli and the
ability to develop neuropathic pain (120). These data are remarkably consistent with gene-deletion studies (103). The sensitivity of
C-MH fibers innervating hairy skin to cold, heat, and mechanical
stimuli is reduced in mice constitutively lacking MrgprD-expressing cells reported to be TRPV1 and peptide negative (121). However, when genetic ablation of this population of cells is done in
adulthood, behavioral deficits were observed to mechanical but
not thermal stimuli (122). An additive loss of both mechanicaland heat-induced nocifensive behaviors was achieved after further
pharmacologic ablation of central TRPV1+ terminals, suggesting
a separation of mechanical and thermal modalities at all levels of
sensory processing in the pain pathways subserved by MrgprDand TRPV1-expressing cells (122). The extent to which this separation is maintained for other murine nociceptor populations is
a subject of active research.
Future challenges
Despite significant progress in understanding the complexities
of mammalian nociception and pain perception in the last half
century, our knowledge is far from complete with regard to the
identity of the full complement of sensors of noxious stimuli
(particularly with regard to mechanotransduction), the role of
nociceptor heterogeneity in physiological and pathological pain,
the coding of the quality of the stimulus, and the modulation of
pain pathways by peripheral and central mechanisms. A focus on
mechanisms underlying thermal nociception and hyperalgesia is
in large part due to the identification of the TRP family of channels. The future identification of elusive mechanotransducers
in somatosensory neurons will likewise thrust the direction of
research toward a cellular/molecular understanding of mechanical
hyperalgesia and allodynia. The application of genetic technologies and pharmacological approaches to understanding the contributions of molecules, signaling pathways, and cell populations to
nocifensive behaviors to particular stimulus modalities in normal
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and pathophysiological states in rodents will inspire hypotheses
that ultimately must be tested in humans.
was supported by NIH grants R01 DE016927 and R01 NS046303
(to A. Patapoutian).
Acknowledgments
We gratefully acknowledge Jorg Grandl, Daniel Dubin, Takashi
Miyamoto, and Arturo Galvez for helpful discussion. The work
Address correspondence to: Adrienne Dubin, 10550 N. Torrey
Pines Road, La Jolla, California 92037, USA. Phone: 858.775.6632;
Fax: 959.784.9107; E-mail: [email protected].
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